Generally speaking, when confidential data is transmitted in a telecommunications network it is standard practice to encrypt the data using a cryptography algorithm. The sender and receiver terminals must then be able respectively to encrypt, and to decrypt the data transmitted. Such cryptography operations require very complex calculations, a long data processing time, high usage of processor and memory resources, and a consumption of electrical power that can prove unsuitable for some sender and/or receiver terminals, for example sensors provided with a radio interface or any other small communications device having in particular limited energy endurance and limited data processing capacities.
Other methods aim to protect the confidentiality of data transmitted between two terminals without using encryption.
Some of those methods send noise data in transmission channels that are not covered by the receiver terminal. If a receiver of a potential hacker receives at least some of the transmission channels not covered by the receiver terminal and carrying noise data, the data captured by the hacker is affected by noise and cannot be used to recover the signal transmitted. The document “Secret Communication using Artificial Noise” by R. Negi and S. Goel, 2005, describes one such method of protecting data during transmission.
However, such methods can be unsuitable in some transmission contexts since they create interference on transmission channels that may be used to transmit other data.
U.S. Pat. No. 2,292,387 describes another method of protecting the confidentiality of a transmission by distributing the data to be transmitted over successive predetermined transmission frequencies. The times at which to change from one frequency to another are chosen beforehand. The sender and the receiver are synchronized by means of two identical punched paper tapes, for example, one used by the sender and the other by the receiver.
However, such methods are suitable only if the transmission frequencies used are available at all times to the sender and receiver concerned (a transmission frequency is “available” if there is no other telecommunications signal being conveyed at that frequency at a given time and in a given geographical location).